A magnetic random access memory (MRAM) is a nonvolatile memory which basically includes a giant magnetoresistive (GMR) material or magnetic tunneling junction (MTJ) structure, a sense line, and a word line. The MRAM employs the magnetic vectors to store memory states. Magnetic vectors in one or all of the layers of GMR material or MTJ are switched very quickly from one direction to an opposite direction when a magnetic field is applied to the magnetic material over a certain threshold or intensity. According to the direction of the magnetic vectors in the GMR material or MTJ, states are stored, for example, one direction can be defined as a logic "0", and another direction can be defined as a logic "1". The GMR material or MTJ maintains these states even without a magnetic field being applied. The states stored in the GMR material or MTJ can be read by passing a sense current through the cell in a sense line because of the difference between the resistances of the two states.
In very high density arrays of magnetic memory cells the size of individual cells becomes very small if the array is constructed small enough to be useful in present day electronic devices. As the size of individual cells becomes smaller the aspect ratio (length/width ratio) generally becomes smaller. In two layer magnetic memory cells, e.g. standard tunneling cells, as the aspect ratio goes below 5, magnetic vectors in the cells are antiparallel in non-energized (zero magnetic field) conditions. In a copending application entitled "Multi-Layer Magnetic Memory Cells with Improved Switching Characteristics", Ser. No. 08/723,159, filed on Sep. 25, 1996, and assigned to the same assignee, methods of reading cells with antiparallel magnetic vectors are disclosed. Also, in a copending application entitled "Magnetic Device Having Multi-Layers with Insulating and Conductive Layers", Ser. No. 08/834,968, filed on Apr. 7, 1997, and assigned to the same assignees, a dummy magnetic layer is added to a two magnetic layer stack and coupled to one of the two magnetic layers so that the other magnetic layer is a free layer. A drawback of the dummy magnetic layer approach is that it relies on cancellation of magnetostatic interaction between the two magnetic layers and this magnetostatic interaction strength depends on the geometry of the cell and the interlayer spacing. These parameters change as the critical dimension shrinks.
Also, in two layer magnetic memory cells, e.g. standard tunneling cells, as the aspect ratio goes below 5, the amount of magnetic field required for switching states of the cell increases dramatically. Generally, as the cells are made smaller the layers of the cells are made thinner to reduce the amount of magnetic field required for switching states of the cell, since the magnetic moment (determined by the material) times the thickness of the layer determines the required switching field. Softer magnetic material can also be used to reduce the magnetic moment but the reduction in switching field is limited for ultra small memory cells. Also, as the cells are made smaller they become unstable because, for example, when the size of the memory cell is 10 nm or less, the energy barrier of the magnetization, which is proportional to the cell volume, is decreased due to the decreased volume and is close to the thermal fluctuation energy, which is KT.
Accordingly, it is highly desirable to provide magnetic random access memories and memory cells which are capable of being written (stored states switched) with less magnetic field and which have a volume sufficient to not be effected by the thermal fluctuation energy.
It is another purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell with ferromagnetically coupled magnetic layers designed to reduce the required switching field.
It is still another purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell with ferromagnetically coupled magnetic layers which can be fabricated very small and with an aspect ratio less than 5.
It is a still further purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell with an increased energy barrier of magnetization.